Use of ultrasound to detect disturbances in a beam path is a well known application in medical devices. Various device manufacturers produce diagnostic ultrasound systems for imaging tissue, utilizing the basic principles of transmitting an ultrasound pulse into the body, then listening for the reflections. Reflections occur when the ultrasound beam crosses a barrier between different types of tissues, such as bone, muscle, fat or organs. Each boundary between a tissue type with different acoustic impedances produces a well known and well characterized type of reflection that allows diagnostic systems to identify objects in the beam path.